The plasma membrane fraction, its two major derivative sub-fractions, and a smooth-surfaced microsomal fraction have been prepared from mouse liver tissue homogenates and their purity has been assessed by using enzymic markers. The antigenicity of the fractions was compared using a lymphocytotoxicity inhibition assay. The results indicate that the vesicular subfraction of the plasma membranes, known to be enriched in certain marker enzymes, was the most active preparation, whereas the microsomal fraction was the least active. An approximately two-fold increase in antigenicity was observed when the assay was performed on deoxycholate-treated fractions. The results suggest that the H-2 histocompatibility antigens may be present at higher concentrations in certain areas of the liver cell surface membrane.